This invention relates to solvent extraction and more particularly to solvent extraction of a solid using two mutually immiscible solvents.
Solvent extraction by two solvents is frequently used in the processing of oilseeds. In a typical process, the seed is extracted with hexane after suitable treatment to isolate triglyceride oil. The residue after extraction contains polar compounds present in the oilseeds which detract from their usefulness. These compounds are generally removed by subsequently extracting the residue with a polar solvent such as alcohol
Methods of extracting soybean meal are disclosed in U.S. Pat. No. 3,878,232 (1975) (Haynes and Simms) and U.S. Pat. No. 3,816,389 (1974) (Mihara et al).
Generally, for oil extraction of oilseeds conventional extractors contact the seeds with extracting solvent by percolating the extracting solvents through the seeds.
The early extractors were of the batch type, and they are still in use for the recovery of oil from oilseeds or mechanical press residues. In modern plants, however, batch equipment is used principally in the form of small units for the recovery of pharmaceutical oils, fish-liver oils, or other expensive oils. A common extractor has been described by Goss (Oil and Soap, 23, 348-354, 1946).
As a result of the shortage of fats and oils after World War I, the Germans sought better ways to extract Manchurian soybeans, and two continuous extractors were developed. The Bollman or basket extractor was patented in Germany in 1919 and 1920 (H. Bollman, German Pat. Nos. 303,846 (1919) and 322,446 (1920); British Pat. No. 156,905 (1921)), and the Hildebrand U tube extractor was patented in 1931-34 (K. Hildebrand, German Pat. Nos. 528,287 (1931) and 547,040 (1932); U.S. Pat. No. 1,961,420 (1934). In recent years the Bollman percolation-type extractors became dominant in the oilseed industry for seeds that have been rolled into thin flakes.
A rotary-type percolation extractor was developed by Blaw-Knox (now Dravo) and called the "Rotocel" (Karnofsky, JOACS, 26, 570-574, 1949, Chem. Eng., 57, 108-110, 1950; McCubbin and Ritz, Chem. Ind., 66, 35-56, 1950). It carries baskets in a rotary motion in a single horizontal plane. Miscella percolates through the baskets and falls into compartments in the bottom of the extractor housing, where it is picked up by a series of pumps and recirculated countercurrently to the flakes. Current models have capacities of up to 3,000 tons per day of soybean. This type of extractor is licensed by Simon-Rosedown and Krupp (Bailey's Industrial Oil and Fat Products, editor D. Swern, 1982, Vol. 1, p. 234).
An alternative design is the French stationary basket extractor, licensed by Speichem (Milligan, JAOCS, 53, 286-290, 1976), in which the liquid manifolds and solid hopper rotate, and the cells and perforated doors are fixed. These also reach capacities of 3000 tons/day for soybean.
The De Smet extractor (Extraction De Smet, S. A., Edegen, Antwerp, Belgium) uses a horizontal endless perforated belt.
The Crown (Crown Iron Works Company, Minneapolis, Minn.) extractor employs percolation combined with immersion. It consists of a chain converyor unit in which a double drag chain and flight move inside a stationary casing, conveying the solids over sections of screen. Some of the units now have the capacity to handle up to 2,000 tons per day of soybeans.
A fifth type of percolation extractor is the FILTREX solvent extraction system, which is of the horizontal rotary filter type. It is made by Wurster and Sanger (Wurster and Sanger, a Division of Jacobs Engineering Company, Chicago, Ill. The advantages claimed for this extractor include low fines in the miscella, superior-quality crude oil, and less solvent to evaporate from the meal (Decossas et al., Ind. Eng. Chem., 49, 930-935, 1957; Haines et al., Ind. Eng. Chem., 49, 920-929, 1957).
A new approach to enhance extraction efficiency is to apply the solvent in vapour form, so as to penetrate the interstices of the material and to condense therein as well as on the external surfaces of the material (Lloyd, British Pat. No. 1,129,165 (1967)). The apparatus consists of a cylindrical container, a disk in the form of a perforated plate, or a mesh, which permits the passage of miscella but prevents the passage of the oil-bearing material. A plough is used to force the material radially on the moving disc to be discharged through an opening in the container wall.
Conventional extractors rely on the integrity of the seed bed for solid-liquid contact, and separation of the phases. They cannot handle finely divided seed material at an acceptable flowrate. Seeds with a high oil content generally need to be prepressed and flaked to be used in conventional systems.
Attempts were made to develop a counter-current leaching process by allowing solid seed particles to move through the solvent by gravity. U.S. Pat. Nos. 2,112,805 (1938), 2,156,236 (1939) and 2,148,248 (1939), all issued to Bonotto, disclose an extractor having a column divided into a number of sections by a revolving assembly of horizontal plates attached to a control shaft. The plates are provided with a series of slots through which the flakes proceed downwardly by gravity, countercurrent to a rising flow of solvent. Stationary scraper arms placed just above each plate provide gentle agitation of the mass to prevent packing and assist in moving the flakes through the holes. In the original patent a screw discharge mechanism is described. For seeds other than soybeans, the discharge mechanism is unsatisfactory and has been replaced generally by a drag-link conveyor.
The Anderson extractor (U.S. Pat. No. 2,663,623 (1953)) was a modification of the Bonotto apparatus. Because of problems with the fine particles in the miscella, it operated more successfully on prepressed cake.
The extraction of oilseeds with solvents that are only partially miscible with oil is the basis of several patents. In these processes the extraction solvent separates from the extracted oil due to increased water content (Cavanagh and Couche, British Pat. No. 1,081,640 (1967) and U.S. Pat. No. 3,295,985 (1966)), or due to decreased oil solubility at decreased temperatures (Youn and Wilpers, U.S. Pat. No. 4,208,540 (1981)).
In the above-mentioned Cavanagh and Couche patent the hydrophilic solvent mixture enters the first of a series of extraction stages as a single phase, and separates into two phases in later stages due to the dissolution of water present in the seed. Although the process gave poor oil recovery and oil quality, it did extract some polar components from the seed in addition to oil. The process has not been used commercially.
The extraction of a soluble component from one solvent into another immiscible liquid is an important unit operation. The simplest form of liquid-liquid contactor allows the droplets of one liquid to flow up or down through a continuous phase of another by gravity. The principle of adding pulsating mechanical energy to increase the degree of turbulence and decrease the droplet size was originated in 1935 by Van Dijck (U.S Pat. No. 2,011,186), who proposed the pulsing of the liquid flow or the introduction of a reciprocating plate into the column. In the early 1950's three types of columns were introduced:
(i) the rotating disc column (Reman and Olney, Solvent Extraction Symposium, Annual Meeting of American Institute of Chemical Engineers, 1954). PA0 (ii) the Scheibel column (U.S. Pat. No. 2,850,362 (1958), U.S. Pat. No. 3,389,970 (1968)). PA0 the Oldshue-Rusthon column (Chem. Eng. Progr., 49,297 (1953)), followed in the 1970's by the Kuhni extraction column.
The reciprocating-plate technique had been neglected until the late 1950's. In 1959 Karr (AICHEJ, 5, 446) reported data on a 76 mm diameter open-type perforated reciprocating-plate column. The column was further developed (Karr and Lo (Chem. Eng. Progr., 72/ 68 (1976)). Interest in this type of column has increased in the past two decades. Most of the published data have lent support to the conclusion that reciprocating-plate columns generally have high volumetric efficiencies. A further development in extractors of this type is the multistage vibrating disk column, introduced by Tojo et al. (Chem. Eng. Sci., 33, 601 (1978)).
It is an object of the present invention to provide an improved method of extracting oilseeds.
Accordingly, the invention provides a process of extracting components from solid particulate matter. The first step of the process is to mix the particles with a first extraction solvent to provide a slurry. The slurry is then passed through an extraction zone. A second extraction solvent is passed countercurrently to the slurry through the extraction zone. The solvents have different densities, permitting countercurrent movement by gravity. The two solvents are substantially immiscible.
In the extraction zone, one solvent flows downwardly and the other solvent flows upwardly through the first solvent due to the specific gravity differences therebetween. The particles are thereby contacted by both solvents simultaneously to remove components therefrom. The particles can be carried by either solvent and are preferably separated from the carrier solvent, and desirable components extracted from the particles can subsequently be recovered from the solvents after extraction, if desired.
Preferably one of the solvents is polar and the other solvent is non-polar. The presence of a polar solvent, often increases the extraction rate of non-polar components into the non-polar solvent.
This process is applicable to any particles which need to be extracted by two immiscible solvents, but is particularly suitable to the extraction of oil-bearing seeds.